Field of the Invention and Related Art
This invention relates to a projection exposure apparatus and a device manufacturing method using the same. For example, the invention is suitably applicable, in the manufacture of semiconductor devices such as ICs or LSIs, image pickup devices such as CCDs, display devices such as liquid crystal panels or other devices such as magnetic heads, to a lithographic process wherein a pattern formed on a mask or reticle is printed or scanningly printed on a wafer through a projection optical system, whereby high density devices are produced.
The present invention is particularly suitably applicable to a case where a reticle and a silicon wafer are registered (aligned) precisely with each other and an electronic pattern on the surface of the reticle is printed on the wafer surface by projection exposure or scanning projection exposure.
For the manufacture of semiconductor devices or liquid crystal panels, for example, on the basis of photolithography, a projection exposure apparatus (called a "stepper") is used by which a pattern formed on the surface of a reticle is transferred by exposure through a projection optical system onto a photosensitive substrate such as a wafer or glass plate having a photoresist coating.
Recent semiconductor technology has been advanced considerably in the point of miniaturization, and resolution for a linewidth of 0.25 micron or narrower is now under discussion. One major technique for this is an optical exposure technique which might be represented by a stepper. Generally, the performance of a projection lens (projection optical system) which may be an index for the performance of an optical exposure process includes three aspects, that is, narrowing of wavelength, enlargement of picture field and enlargement of numerical aperture (NA). In relation to bandwidth narrowing, development of lithography that uses light of an ArF excimer laser has been attempted widely, as the next generation technology.
For the optical exposure technique, there is a factor that the same optical performance should be held continuously. In the case of lithography that uses an ArF excimer laser, there is absorption of light by a usable glass material, and it is known that the optical performance of a projection optical system is variable due to such absorption of light. The absorption of light is already known even in the case of lithography that uses light of i-line, for example. Since a stepper having an ArF excimer laser has a decreased depth of focus of a projection optical system, the optical performance should be controlled more precisely than before. Of such optical performance to be controlled, there are aberrations of a projection optical system. Those of these aberrations which are most difficult to be corrected are axial (axial) astigmatism, curvature of image field, and third-order (cubic) or higher-order distortion. Further, correction of revolutionally asymmetric magnification of an object itself may be an additional problem to be solved.
Axial astigmatism will be explained below, with reference to an example. For enlargement of a region on the wafer surface to be exposed, development of a scan type projection exposure apparatus (called a "scanner") has recently been made widely, wherein an illumination region of a slit-like shape is defined and wherein a reticle and a wafer are scanningly moved relative to the slit shape, in synchronism with each other. When such slit-like exposure light is used, since the shape of the slit is not revolutionally symmetrical, absorption of light by the glass material may cause an asymmetric thermal distribution with respect to the optical axis which may result in astigmatism (axial astigmatism) of the projection optical system.
Next, revolutionally asymmetric magnification will be explained. As one factor required for an optical exposure technique for the manufacture of semiconductor devices, in addition to improvement of resolving power, there is registration (alignment) precision for patterns to be superposed one upon another in several layers.
A frequently used alignment process is one called a "global alignment method". Errors in such a global alignment procedure may be classified generally into two, that is, an inter-shot component (an error between different shots) and an intra-shot component (an error within one shot). Because of recent enlargement of picture field size, how to reduce the error of the intra-shot component is now a problem to be considered. For an actual wafer, there occurs asymmetrical distortion in dependence upon the process adopted.
For example, if, for a picture field size of 22 mm, there is an error of 2 ppm in terms of magnification, as a component which is asymmetric and cannot be corrected, it means that there is an error of 22 mm.times.1 ppm=44 nm. For resolution performance of 0.25 micron linewidth, such error has a value of about 1/5.times.. From the standpoint of registration budget, clearly such a value is out of tolerance. Thus, to reduce the intra-shot component is an important problem to be solved in optical exposure apparatuses.
As regards distortion, there are known measures to control magnification and third-order distortion. An example is that plural elements within a projection optical system are displaced along the optical axis direction, or the pressure of a gas sealingly provided between optical elements is changed. Since magnification is a fundamental quantity of an optical system, it can be changed without changing the other aberrations. As regards correction of third-order distortion, however, there is a problem of changes in aberration resulting from displacement or a problem of a small range of adjustment. Thus, correction has to be taken into account even in the initial stage of design. Particularly, in consideration of the use of different imaging methods such as a modified illumination method and a phase shift mask, for example, there will occur a problem of matching of distortion in these imaging methods. It is, therefore, important to control third-order distortion without any additional burden to design.
Higher-order distortion will now be explained. In a scan type projection exposure apparatus, an asymmetric magnification difference such as, for example, a magnification difference between X and Y axes which are in directions perpendicular to the optical axis can be corrected by the scanning. Also, as regards the scan direction, distortion may be averaged due to the averaging effect resulting from the scanning. For example, if the slit is elongated in the X direction and the scan is made along the Y-axis direction, the result of scanning is that the magnification in the Y direction can be adjusted by controlling the synchronism of scan, and that optical distortion of third-order or fifth-order, for example, can be suppressed to a small value due to the averaging effect within the slit. As regards the X direction, however, there is no averaging effect. It is, therefore, necessary to optically control the distortion with high precision.
Japanese Laid-Open Patent Application, Laid-Open No. 183190/1995 shows a projection exposure apparatus wherein a revolutionally asymmetric optical characteristic remaining in a projection optical system, for projecting a pattern of a mask onto a wafer, with respect to an optical axis of the projection optical system is made adjustable.
In the arrangement of the aforementioned Japanese Laid-Open Patent Application, Laid-Open No. 183190/1995, when a cylindrical lens having a revolutionally asymmetrical power is driven, it produces variations in plural aberrations. Therefore, it is difficult to correct only an aberration which is just to be corrected.